Since the beginning of the 20th century, the growing season in many areas of the lower 48 states has expanded by about two weeks. Frosts end earlier in the spring and begin later in the fall. To gardeners in Maine, Wisconsin and Montana, that might seem a blessing. What’s not to like about more lettuce or riper tomatoes?

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The longer seasons, however, are also helping invasive plants annex American soil; extended springs mean they can more quickly push aside native species and transform ecosystems. “What’s interesting about climate change is that humans are effectively manipulating how species experience time,” says ecologist Elizabeth Wolkovich of the Biodiversity Research Centre at the University of British Columbia.

Wolkovich and her colleagues have been studying how the first flowering dates of plants have changed over the years in Kansas, North Dakota, Washington, D.C., Concord, Massachusetts—where Henry David Thoreau kept notes about the flowers blooming near Walden Pond in the mid-1800s—as well as Britain.

The researchers found that many nonnative species are changing their flowering schedule in concert with the longer growing season. For example, purple loosestrife—a European import widely planted in the 19th century for medicinal use—blooms 24 days earlier in Concord than it did a century ago. By contrast, indigenous species have been, on average, much slower to react to the new conditions: Pennsylvania bitter cress, a familiar native plant in Concord, blooms only about a day earlier than it did in the early 1900s.

That trend worries scientists, given that invasive plants already cost the United States around $30 billion per year in eradication efforts, not to mention the damage to biodiversity and natural habitats. Purple loosestrife, for instance, chokes wetland habitats by crowding out cattails and other marsh plants that serve as food, shelter and nesting material for wildlife.

Why do indigenous plants lose ground to exotic species? Botanists have learned that several invasive species are, by nature, highly flexible, and respond to unusual environments more quickly than do natives. And now, with the help of climate change, the invasives also reap the benefits that come with early blooming—such as shading out competitors and capturing a larger share of nutrients, water or pollinators.

“It’s shocking to see how quickly the playing field is being shifted in favor of species that can be super-adaptable,” says Wolkovich. “The species that win are going to be those that can take advantage of new opportunities very quickly. And I don’t think natives will often be among those species.”